This invention relates to voltage controlled oscillators and, more particularly, to a gain compensated varactor tuning network for a voltage controlled oscillator.
Voltage controlled oscillators (VCO's) are oscillators which have a frequency output which may be varied in response to an applied voltage signal. VCO's are typically employed as a component within a phase-locked loop (PLL) circuit. A PLL is typically utilized when it is desired to synchronize two signals in both phase and frequency. As an example, one signal may be a local oscillator within a communications device, while the second signal may be a received frequency.
A phase-locked loop may be characterized as a closed-loop electronic servomechanism the output of which locks onto and tracks a reference signal. Phase lock is achieved by comparing the phase of the PLL output signal to the phase of the reference signal. Any phase difference between the two signals is converted to a correction voltage, typically by a phase detector circuit. This correction voltage is applied to the VCO circuit, thereby adjusting its output frequency such that it tracks the reference frequency.
As may be appreciated, the response time of the VCO circuit to a change in the applied correction voltage is an important parameter of such a circuit, especially if the reference frequency is varying rapidly. As may be further appreciated, the control of the VCO's tuning characteristic, that is the ability of the VCO output frequency to vary in a controlled and predictable fashion with respect to a change in the correction voltage, is also an important consideration.
Achieving a specific VCO voltage-to-frequency relationship is especially difficult when the VCO uses one or more varactor diode components as frequency tuning elements. Varactor diodes are essentially two terminal semiconductor diodes wherein the inherent p-n junction capacitance is emphasized. By varying a reverse bias applied to such a varactor diode, the junction capacitance may be varied, thus making the device equivalent to a voltage controlled capacitor. By their inherent nature however, varactor diodes have an undesireable nonlinear voltage-to-capacitance relationship, that is, the capacitance varies in a nonlinear fashion with respect to the applied reverse bias voltage. Furthermore, the particular nonlinear characteristic of a varactor diode is dependent, typically, on certain specific physical characteristics of the diode. When such a varactor diode is utilized as a tuning component in a VCO, the resulting output frequency of the VCO likewise exhibits an undesirable voltage-to-frequency relationship, the frequency being dependent on the capacitance of the varactor diode. In response to the problem created by this inherent voltage-to-capacitance relationship of a varactor diode, it has been known in the prior art to utilize various linearization techniques to compensate for the undesireable nonlinearity characteristics of the varactor diode.
One such technique employs a high speed linearizing driver circuit to provide a linearized correction voltage to the VCO. Such a driver circuit is typically comprised of a high slew-rate operational amplifier combined with a breakpoint generator, the generator acting to change the gain of the amplifier at preset "breakpoints" as the input correction voltage varies. The resulting output signal is thereby composed of a number of linear segments, the number of such segments increasing with an increasing number of breakpoints of the generator. The output linearized correction voltage is thus made to vary in a nonlinear fashion with respect to the applied correction voltage. The response time of the circuit with respect to a change in the applied correction voltage is typically in the range of 50 to 500 nanoseconds. The effect of the linearized correction voltage is to compensate for the nonlinear voltage-to-capacitance characteristic of the varactor tuning components, thereby providing a VCO having an output frequency which varies in a more linear fashion with respect to the correction voltage.
A disadvantage of such a linearing circuit is that, in order to achieve a minimum response time to a change in the correction signal, a high-slew rate and, hence, expensive operational amplifier is required. Another disadvantage of such a circuit is that the power consumed by components of the breakpoint generator, such components typically being transistors and resistors, varies with respect to the operating frequency. Hence, the thermal time constants and thermal stability of these components are critical factors which affect the short term frequency stability of the VCO.
While the linearizing circuit as described above is adapted for use with an input analog correction voltage, it may also be utilized in a PLL which employs digital components within the feedback loop between the VCO output and the phase detector. Typically, a digital counter is utilized to divide the output frequency of the VCO. The outputs of the digital counter are applied as inputs to a digital to analog converter (DAC), whereby an analog correction voltage is produced for application to the breakpoint amplifier of the linearizing circuit. As may be appreciated, the signal delay incurred by the operation of the DAC is additive to that of the linearizer, with the resulting overall response time of the DAC linearizer combination being in the range of approximately 500 to 2000 nanoseconds.
Because of the aforementioned problems of high cost and sensitivity to thermal effects associated with the breakpoint type of linearizer, it has been known in the art to replace the breakpoint linearizer with a digital look-up table, the look-up table being disposed between the output of the counter and the input to the DAC. The look-up table is contained typically within a high speed programmable read only memory (PROM). The PROM is configured such that its address inputs are connected to the outputs of the counter. The data outputs of the PROM are connected correspondingly to the digital inputs of the DAC. In operation, the outputs of the counter address a word of data within the PROM, the PROM thereafter outputting on its data lines the data so addressed. The value of each word of data contained within the PROM is determined by a calibration procedure performed when the PLL is first constructed. During the calibration procedure, the counter is driven such that it will output all possible combinations of PROM addresses. For each such address a corresponding PROM output is determined which, when applied to the DAC, produces an analog correction voltage which provides a linear VCO voltage-to-frequency transfer function. The digital value so determined is stored in the PROM at the addressed word. Thus, it can be seen that the aforementioned analog breakpoint linearizer is replaced with a digital look-up table contained within the PROM, the PROM providing a number of equivalent breakpoints that correspond to the number of words within the PROM. For example, if the PROM has 12 address inputs, the VCO will be linearized at the equivalent of 2,048 breakpoints.
While the aforementioned prior art table look-up scheme is suitable for modifying the voltage-to-frequency characteristics of a VCO utilizing varactor diode tuning elements, it is also disadvantageous for several reasons.
One problem created by the table look-up approach is that an additional response delay is introduced by the data access time of the PROM. This delay is additive to the delay of the DAC and results in a minimum response time delay of approximately 1000 nanoseconds.
Another problem created by this approach is that in each PLL system incorporating such a table look-up PROM, the system must be individually calibrated and the PROM custom programmed in order to compensate for the intrinsic characteristics of the VCO components, especially the nonlinearity characteristics of the varactor devices incorporated therein. This requirement for individualized system calibration is time consuming and costly, especially in relatively high production environments. An additional problem created by this approach is that if a system component is required to be changed at a later time, such as during field use, it may be necessary to recalibrate the system and program a new PROM in order to compensate for the different characteristics of the new component introduced into the system.